Map Picture UNO property on content controls to:
<loext:content-control loext:picture="...">
And do the opposite on import.
Change-Id: I47c3d04c505ba94da958d5d6b6b2c883236afc3d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/134595
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>
